1. Field of the Invention
This invention relates to an improvement in image-receiving sheets for thermal transfer printing which ar adapted for printing images thereon by transfer of a sublimable dye of a thermal transfer sheet by application of heat from a thermal head in accordance with image signals.
2. Description of the Prior Art
Thermal printing systems in which printed images are obtained upon reception of input signals are made up of a relatively simple apparatus and are inexpensive and low in noises, so that they have increasing utility in various fields such as facsimiles, terminal printers for electronic computers, printers for measuring instruments, video printers, and the like.
A recording medium generally used in these thermal printing systems is a so-called color-developing, heat-sensitive recording paper having a recording layer which undergoes a physical or chemical change by application of heat to form a color. However, the recording paper of the color developing type has disadvantages in that it is liable to undesirably develop the color during the process of fabrication or during storage. In addition, the image printed on the paper is poor in storage stability and is apt to fade on contact with organic solvents or chemicals. To avoid this, there has been proposed a thermal transfer printing system in which a recording medium utilizing a dye or colorant is used instead of the color-developing thermosensitive recording paper.
In this thermal transfer printing system, a colorant or colored dye is caused to melt, evaporate or sublimate by application of heat and transferred on a recording paper, thereby forming a record image by adhesion, adsorption or reception of the dye on the recording paper. For instance, there has been proposed a mechanism as is shown in FIG. 2. In the mechanism, a thermal transfer sheet 201 having a dye layer on a substrate and a thermal transfer image-receiving sheet 203 set on a platen roller 202 are superposed and heated from the non-faced side of the thermal transfer sheet 201 by a heating means 204 such as a thermal head. The thermal head 204 is controlled with electric signals corresponding to image information. As a result, the dye of the thermal transfer sheet is transferred on an image-receiving layer.
For full color printing, a thermal transfer sheet used is a sheet which has been coated or printed, as shown in FIG. 3, a start mark 301 and dyes including yellow 302, magenta 303 and cyan 304, and also including a black dye, if necessary. This sheet is superposed on a thermal transfer image-receiving sheet and the yellow, magenta and cyan dyes are successively heated according to the respective color-separated image signals to make an image in which the three colors are superposed. Thus, a natural color, photographic image is formed.
A known thermal transfer image-receiving sheet used in this type of printing system is one which has an image-receiving layer obtained by coating onto a paper substrate such as high-quality paper a coating of a dispersion of finely powdered silica or calcium carbonate in a binder such as a thermoplastic polyester resin.
However, when the image-receiving layer is formed on the substrate, such as wood free paper, whose smoothness is low, it is difficult to obtain a satisfactory printed image. Especially, where an image with a degree of gradation is transferred, missing transfer portions are liable to occur at half-tone to low-tone portions. Although images such as of symbols, letters and figures are substantially solid and thus the missing transfer portions are not conspicuous, missing transfer portions on photographs or colored solid images will undesirably tend to become marked and produce vital deficiencies. Accordingly, there is a strong demand for eliminating such deficiencies.
In reply to said demand, various improvements have been proposed including, for example, (1) a method proposed in Japanese Laid-open Patent Application No. 61-172795 in which an intermediate layer is formed beneath an image-receiving layer to improve a smoothness on the surface of the image-receiving layer, and (2) a method proposed in U.S. Pat. No. 4,720,480 in which an intermediate layer made primarily of a flexible resin is formed.
However, these improvements are still accompanied with other problems and thus, satisfactory results cannot always be obtained. For instance, the method (1) has a problem that the productivity lowers because it comprises the step in which the intermediate layer which has been kept as softened or plastic is brought into close contact with mirror-like finished chromium plated metal drum for heating and drying. The method (2) involves the problem that a soft resin such as MBR, polyurethane, polybutadiene, SBR or the like, which has been formulated so as to improve the adhesiveness, is liable to block when wound up after coating and drying. Additionally, the resin dissolves in an organic solvent of the coating solution forming an image-receiving layer, so that the image-receiving layer cannot be formed uniformly, thus causing a lowering of an optical density of the printed image.